London Irish were dragged deeper into the relegation mire after champions Harlequins ground out a 26-15 victory in front of 82,000 fans at Twickenham.
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The Exiles were in touch for 78 minutes but left empty-handed after a late penalty try from Harlequins' dominant pack denied them a losing bonus point.
Harlequins had taken the lead after a scrappy first half with a try from scrum-half Danny Care and Nick Evans added 16 points with the boot.
London Irish are now just one point ahead of the Aviva Premiership's bottom side Sale Sharks, who closed the gap with their victory over Worcester on Friday.
The Sharks are in a state of upheaval after coach John Mitchell announced today he was returning to South Africa prematurely after just a month at the club.
But the Exiles have just two Premiership wins all season and they are not in a position to be relying on Sale shooting themselves in the foot.
Harlequins, meanwhile, finish a memorable 2012 top of the table.
Their record in this annual Christmas extravaganza at Twickenham has not been great, with just one victory from the four previous games.
But Conor O'Shea's men do not lose at Twickenham anymore.
Harlequins won their first Premiership title here in May and opened the new campaign with a stunning comeback victory over Wasps in the London double header.
There was nothing stunning about this victory, which was ground out in miserable conditions.
Harlequins did have the crowd on its feet in the opening minutes as they exploited the Exiles' notoriously weak defence, moving the ball quickly before Joe Marler charged through the gap.
The England prop was hauled down but the Quins pack kept the pressure on and earned a penalty which Evans converted after Exiles captain Declan Danaher slipped his binding.
Evans angled an intelligent kick into space behind the London Irish defence as Harlequins threatened to take control, but the Exiles were resilient.
Chris Hala'ufia carried the ball strongly and London Irish drew level with the first of Ian Humphreys' five penalties as the heavens opened.
The game worsened for it and became a scrappy affair, with both sides committing handling errors. Evans was the chief culprit for Harlequins, losing possession three times in the first half.
The Kiwi fly-half did kick his side back into the lead after Exiles prop Max Lahiff was penalised under pressure from James Johnson as Harlequins bossed the scrum battle.
London Irish, though, were gifted the opportunity to respond immediately when Maurie Fa'avasalu was penalised at a ruck straight from the kick-off and Humphreys accepted it.
Evans lit up the Twickenham gloom with one delightful reverse pass and then burst through a gap, only to spill the ball in the tackle.
Care had zipped across the field from a powerful lineout drive and offloaded to Evans, who went himself and should have lofted a pass out wide to Ugo Monye before he was tackled.
Harlequins produced another big scrum under the Irish posts but Fa'avasalu was penalised for playing the ball with the foot and the Exiles were able to clear.
Evans and Humphreys each missed two shots at goal before the interval and then nudged the score on to 9-9 after the restart when Harlequins began to up the tempo.
Monye coasted past Humphreys with ease and aimed for the corner before trying an inside ball to Mike Brown, which was knocked on by Humphreys.
Harlequins won another scrum penalty, went for touch and Care launched a 40-metre diagonal attack from the driving lineout before passing to Evans, who was hauled down five metres short.
Care darted for the line from the base of the ruck. He was half stopped by Jonathan Joseph but stretched through a forest of legs and the try was eventually awarded by television match official Geoff Warren.
Irish battled hard in the hope of a bonus point, with Humphreys slotting two more penalties, either side of one from Evans, to his side within four points.
But scrum-half Pat Phipps was sin-binned with four minutes remaining as his scrum struggled in their own 22.
Harlequins went for the scrum again and demolished the set-piece to earn the penalty try and deny their opponents the losing bonus point.